Microfabricated torsional cantilevers for sensitive force detection

ABSTRACT

A torsional cantilever is microfabricated for reduced size to increase its resonance frequency, increase its scanning speed, and permit fabrication of large numbers in an array to provide parallel scanning. The cantilever may incorporate a tip for highly sensitive force detection. The device preferably includes a cantilever arm and a counterbalance mounted on opposite sides of a laterally extending torsional beam fixed at its outer ends. Sensors detect rotation of the cantilever arm and may provide control of sensor locator through a feedback loop.

This application claims the benefit of U.S. Provisional Application No.60/001,296, filed Jul. 20, 1995, the disclosure of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

The present invention relates, in general, to torsional cantilevers, andto methods for fabricating such cantilevers. More particularly, theinvention relates to a high aspect ratio, single crystal silicontorsional cantilever having a cantilever moment arm mounted on atorsional support beam, to arrays of such cantilevers, and to a processfor fabricating such a cantilever which is compatible with processes formaking conventional silicon integrated circuits.

Since their development in the early 1980s, scanning probe microscopeshave become important tools for surface analysis and surfacemodification. The unique applications of scanning probe microscopesinclude imaging and manipulating single atoms, measuring forces on theatomic scale, and performing nm-scale lithography. At the center of thefamily of scanning probe microscopes are the scanning tunnellingmicroscope (STM) and the atomic force microscope (AFM). Thesemacroscopic scanned probe instruments, for the most part, use largepiezoelectric actuators to position a sensing tip or a probe in threedimensions (xyz) and thus to provide relative motion between the tip anda sample surface. However, the size of these microscopic instrumentslimits their performance, for the mass of the tip-actuator structureproduces low resonant frequencies and low scanning rates. Moreimportantly, these large instruments cannot be easily integrated intoarrays for high speed scanning and atom manipulation, for informationstorage, or for high throughput, nm-scale lithography systems.

A cantilever of some sort is often used in macroscopic force microscopyto monitor the variations in forces which represent the interactionbetween a tip and a sample. In such cases, the cantilever is usually asilicon nitride "V" cantilever which, for example, may be 0.6 to 2micrometers thick, may be 100 to 200 micrometers long, and which mayhave a spring constant of between about 0.03 and 3 N/m for contact modeimaging. See, for example, T. R. Albrecht et al, J. Vac. Sci. Technol.A8, 3386 (1990). For non-contact mode imaging, the cantilever may besilicon with a spring constant of about 1 to 100 N/m, as described byWolter et al, J. Vac. Sci. Technol. B9, 1353 (1991). In order to obtaina high degree of sensitivity, a low spring constant k and a high Q isneeded for such cantilevers, and attempts have been made to accomplishthis through the use of thin films. However, it is necessary to make thecantilever very thin in order to achieve a low spring constant with athin film; for example, magnetic resonance force detection has beenperformed using a cantilever (without an integrated tip) that was only900 Å thick, and which had a spring constant of 10⁻³ N/m. However, thefabrication and use of such thin cantilevers poses many problems,including the problem of tip integration, problems with internal stress,and problems in making electrical connections and in amplifying theresulting signals.

For many years, torsion has been used as a technique for highlysensitive measurements of force interactions; for example, measurementof Coulomb's torsional balance for electrostatic forces and Cavendish'sbalance for gravitation. Furthermore, torsional resonators have beenused as high-Q resonators to study a variety of physical properties suchas dissipation and visoelasticity. Such devices have been widely usedbecause cantilevers, resonators or balances can be made symmetric withrespect to their center of mass, with the result that lateral vibrationsof the support do not couple to the torsional mode of the sensor device.In the case where the measuring device includes a spring, lateral modesof the spring can be made much stiffer than the torsional mode withoutaffecting the torsional behavior, thus making torsional measurementsdesirable.

Torsional cantilevers are known to provide a viable option in scanningforce microscopy. However, such cantilevers have, in the past, beenassembled by hand, using cleaved pieces of silicon, carbon fibers ortungsten wire, and epoxy. Such devices did not incorporate integratedtips and, although microfabricated cantilevers have been demonstrated,such devices exhibited a lateral stiffness which was as soft as, orsofter than that of a V-shaped cantilever. The lack of sufficientlateral stiffness can result in unwanted "stick-slip" behavior as thecantilever is scanned across samples.

SUMMARY OF THE INVENTION

The present invention is directed to an improved torsional cantileverwhich is microfabricated for reduced size to increase its resonantfrequency and thereby increase its scanning speed, and to permit thefabrication of large numbers of devices in a unit area to provideparallel scanning of a surface. This microfabricated cantilever mayincorporate an integrated tip so that it is particularly adapted toprovide highly sensitive detection of forces. Further, the cantilever isintegral with a substrate which may incorporate conventional integratedcircuits to which the cantilever and/or tip may be electricallyconnected. In the preferred form of this invention, the substratematerial is a single crystal silicon, and the cantilever structurepreferably is fabricated as a microelectromechanical (MEM) device by thesingle crystal reactive etch and metalization (SCREAM-I) processdescribed in copending U.S. patent application Ser. No. 08/312,797 filedSep. 27, 1994, now U.S. Pat. No. 5,719,073 of Kevin A. Shaw, Z. LisaZhang and Noel C. MacDonald, the disclosure of which is herebyincorporated herein by reference (Docket No. CRF D-1307C) . Thisfabrication process allows integration of the cantilever structure on asingle crystal silicon substrate carrying pre-existing integratedcircuits, and allows the cantilever to be formed with an integralsubmicron tip for use, for example, in scanning surfaces to be measuredin scanning probe microscopy, or in other applications.

The process also permits incorporation of motion control devices such ascapacitors in the cantilever device for producing and/or for controllingscanning motions in x, y, and z directions with respect to the axis ofthe cantilever, with such capacitors also being available to measure tipmotion in response to forces to be detected. The process of the presentinvention also permits incorporation of other sensors, such aspiezoresistive devices, diodes or transistors within the cantileverstructure for use, for example, in detecting motion or in amplifyingelectrical currents detected by the tip. The integration of all of thesefeatures produces a scanning torsional cantilever that is extremelycompact, which requires no external deflection sensor, and which issuitable for use in a variety of atmospheres and temperatures, as wellas for use in dense array architectures of scanned probe instruments forinformation storage and molecular or atomic manipulation.

The torsional cantilever structure is a basic building block formicromechanical scanning probe microscopes, and can be fabricated withan integrated tip in a 100 μm×100 μm square area. The structure includesa torsional support beam which is secured at each of its ends to asubstrate; preferably, the support beam is integrally formed with thesubstrate. A sensor arm, or cantilever moment arm, consisting of a rigidgrid of single crystal silicon beams is microfabricated integrally withthe support beam, and extends generally horizontally outwardly from thebeam in cantilever fashion, the beam support and sensor arm lying in ahorizontal x-y plane and perpendicular to each other. The outermost, orfree end of the sensor arm carries a sensor tip which interacts with asample to be measured.

The cantilever may be made to pivot about its support beam by electrodesfabricated on the substrate beneath the cantilever structure.Alternatively, the structure may incorporate interdigitated combcapacitor structures to provide controlled motion of the cantilever.This pivotal motion of the cantilever about the torsional support beamprovides the out-of-plane motion required to enable the tip to track asurface. If desired, a small xy scanner may be integrated into thecantilever structure for precise lateral positioning of the tip. Forcesapplied to the tip, for example in the vertical (z) direction, deflectthe tip, and this motion is transferred by the sensor arm to the supportbeam in the form of a torsional force applied about the horizontal axisof the support beam. The torsional force applied to the support beam bythe cantilever moment arm can be measured in a variety of ways toprovide an accurate measure of the tip force. For example, the relativemotion of interdigitated capacitor plates, or the motion of thecantilever with respect to an underlying electrode can be measured as achange in capacitance. Deflection can also be measured by apiezoresistor produced in the support beam, as by implanting an n-typeregion into a p-type substrate or implanting a p-type region into ann-type substrate. The doped region forms a resistance which changes dueto stress produced in the support beam by relative rotation of thetorsion arm.

The measurement of rotation can also be accomplished by anotherstress-sensitive device based on the piezojunction effect. In this case,a sensor diode is formed by implanting an n-type region in the p-typesubstrate silicon at the junction of the torsional support beam with afixed mesa on which the beam is mounted. Contacts are made to thisimplanted region and to the adjacent substrate. Stress in the supportbeam causes a change in current through this diode, and changes in thecurrent can be measured.

The cantilever moment arm structure of the invention can be utilizedwhenever out-of-plane or rotational actuators or force sensors arerequired. Such an actuator may be used, for example, in anaccelerometer, wherein an asymmetric design, with more mass on one sideof the support beam than on the other, provides a device which will besensitive to acceleration in directions out of the plane of thecantilever and the substrate on which it is mounted. The actuator canalso be used in force microscopy employing a symmetric design whichbalances the applied force so that the device will be highly sensitiveto changes in a force acting on one side of the cantilever, and not onthe other.

By integrating a tip on the free end of the cantilever moment arm in amicroscope, the torsional cantilever becomes a tip-to-surface forcecenter in a scanning force microscope. The out-of-plane motion, ortorsional cantilever motion (in the z direction), also enables the tipto track surface topography. If desired, the cantilever may carry amirror or an interferometer to provide accurate measurement ofout-of-plane or rotational motion, or one or more of the sensorsdescribed above can measure and/or control this motion.

With the torsional cantilever beam established as a building block, anarray of beams can be fabricated, wherein cantilevers can be provided at100 μm intervals to provide, for example, a 10×10 array of cantileversin a 1 mm by 1 mm square area. Such an array allows an entire 1 mm by 1mm sample to be scanned for imaging or manipulation by moving thesupport mechanism only 100 μm in x and y directions.

The array can be made in several configurations. For example, torsionalcantilevers can be made individually movable in the z direction whilethe sample is scanned in x and y directions. Alternatively, thecantilevers can be fabricated to be movable in the z direction as partof a micromechanical structure that is scannable in the x direction,with the sample being scannable in the y direction. In anotherconfiguration, the z cantilevers can be fabricated as part of amicromechanical stage that is scannable in x and y directions, with thesample remaining stationary. The sample can be a chip with a baresurface or a compliant surface, or can be an active MEM device.Alternatively, the sample can be placed on or bonded to a chip whichcarries an array of microscanned probe devices, wherein the torsionalcantilever elements which make up the probe provide enough out-of-plane(or z-direction) motion to allow the tips to contact the surface. Tofacilitate this, sample support pillars or posts are fabricated on thechip which carries the array, with the pillars having the same heightas, or being a little taller than, the tips.

A large number of electrical connections are required in an array ofcantilevers to carry control signals for individually moving thecantilever arms or for carrying sensor output signals from each arm.Transistors for switching among various devices can reduce the number ofconnections, and such switches can be in the form of normal planar ICtransistors fabricated on the silicon substrate. Metal pathways alongthe substrate provide the necessary connections. Suspended transistorsin the silicon beams can also be used for switching, amplification, andlogic, and electrical isolation can be provided by oxide segments in thebeams.

The cantilevered arms and arrays of such arms can be used in a widerange of applications. In addition to scanning of images and themanipulation of surfaces at atomic and near-atomic scales, the devicesand arrays of the invention can be used for high density informationstorage, in the range of a terabit per cm², for molecular manipulationinstruments, for field emission (e-beam) instruments, and formicrolithographic or micromachining tools.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing, and additional objects, features and advantages of thepresent invention will become apparent to those of skill in the art fromthe following detailed description of preferred embodiments thereof,taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a partial perspective view in diagrammatic form of a torsionalcantilever device in accordance with the present invention;

FIG. 2 is a top plan view of the cantilever device of FIG. 1;

FIGS. 3(a) through 3(h) illustrate a process for fabricating sensor tipson the device of FIG. 1;

FIGS. 4(a) and 4(b) are diagrammatic illustrations of a metallizationprocess used in the device of FIG. 1;

FIG. 5 is a partial, enlarged view of the cantilever structure of FIGS.1 and 2;

FIG. 6 is an enlarged, partial view of the torsional support beam forthe device of FIG. 1;

FIG. 7 is a graphical illustration of curves representing therelationship of effective spring constant to cantilever length fordifferent cantilever cross-sectional widths "a" each having a cantilevercross-sectional height "b" of 10 μm;

FIG. 8 is a top plan view of a second embodiment of the cantileverdevice of the present invention;

FIG. 9 is a top plan view of a third embodiment of the cantilever deviceof the present invention, utilizing torsion control electrodes;

FIG. 10 is a cross-sectional view taken along lines 10--10 of FIG. 9;

FIG. 11 is a fourth embodiment of the cantilever device of the presentinvention, utilizing comb-type capacitors for motion sensing andcompensation;

FIG. 12 is a fifth embodiment of the device of the present invention,utilizing comb-type capacitors;

FIG. 13 is a partial, enlarged perspective view of a modified combcapacitor for the device of FIG. 12;

FIG. 14 illustrates a first alternative version of the comb capacitor ofFIG. 13;

FIG. 15 illustrates a second alternative version of the comb capacitorof FIG. 13;

FIG. 16 is a sixth embodiment of the device of the present invention,combining the features of FIGS. 9 and 10;

FIG. 17 is a diagrammatic illustration of the use of the torsionalcantilever device of the present invention in an atomic forcemicroscope;

FIG. 18 is a photomicrograph of the torsional cantilever device of thepresent invention incorporated in an capacitor-driven xy stageconfiguration;

FIG. 19 is a diagrammatic illustration of an array of cantileveredsensors mounted for motion in an xy plane, with individual cantileversbeing movable in the z direction;

FIG. 20 is an enlarged, partial top plan view of a torsional cantileverdevice incorporating piezoresistive sensors;

FIG. 21 is a cross-section along lines 21--21 of FIG. 20;

FIG. 22 is an enlarged, partial top plan view of a torsional cantileverdevice incorporating diode sensors; and

FIG. 23 is a cross-sectional view taken along lines 23--23 of FIG. 22.

DESCRIPTION OF PREFERRED EMBODIMENTS

Turning now to a more detailed consideration of the present invention,there is illustrated in FIG. 1 a perspective view of a microfabricated,highly sensitive force detecting torsional cantilever device 10incorporating an elongated torsionally-mounted microelectromechanical(MEM) cantilever moment arm generally indicated at 12. This device has awide range of applications, but will be described herein for convenienceas being used as a force sensor in scanning probe microscopy. Thecantilever moment arm portion 12 is mounted at a first, or near end 14to the center of a laterally extending torsional support beam 16. In theillustrated embodiment, the opposite ends of the beam 16 are supportedon, and preferably are integral with, a substrate 18 as at support mesas19 and 20. The arm 12 is integral with beam 16 and in its preferred formis formed of a grid of longitudinal and lateral beams, as illustrated inFIG. 1, the arm extending from its near end 14 generally horizontallyforwardly from the mounting beam 16 to a far, or distal, end 21 along alongitudinal axis 22 (FIG. 2) which is perpendicular to beam 16. Anupwardly-extending, generally vertical, nanometer-scale sensing tip 23is formed on the distal end 21 of the cantilever moment arm in thepreferred form of the invention. This sensing tip is generally conical,tapering upwardly and inwardly to an extremely small diameter, on theorder of one atom, at its end 24. The arm 12 and beam 16 lie in an x-yplane, with tip 22 extending perpendicularly to the plane, in a zdirection.

Also mounted on lateral support beam 16, but on the opposite sidethereof from arm 12, is a counterweight 26 which is also fabricated as agrid of longitudinal and lateral beams (not shown in FIG. 1), and whichis of sufficient size and weight to produce a counterbalancing mass forthe cantilever arm so as to make the arm more sensitive to, andresponsive to, forces applied to tip 23. The counterweight 26 serves asan extension of cantilever moment arm 12 so that arm 12 andcounterweight 26 rotate together about a lateral axis 27 of the beam 16.Axis 27 lies in the x-y plane of arm 12 and beam 16, with axis 27 beingperpendicular to the longitudinal axis 22 of arm 12. Rotation of arm 12and weight 26 occurs upon application of vertical forces in thedirection of arrow 28 in FIG. 1; that is, upon application of forces inthe z direction to the tip 23.

The torsional cantilever device 10, which includes the moment arm 12,the lateral torsional mounting beam 16, the tip 23 and the counterweight26, is fabricated by the SCREAM-I process described in the aforesaidU.S. application Ser. No. 08/312,797, now U.S. Pat. No. 5,719,073discussed above. This process includes a reactive ion etching processfor fabricating suspended, released, single crystal siliconmicroelectromechanical (MEM) structures, with feature dimensions assmall as 250 nm and with arbitrary structure orientation, from thesingle crystal silicon wafer or substrate 18. During the fabricationprocess, the torsional cantilever structure 10 is released from theunderlying substrate so that it is mechanically spaced from, issupported over, and is generally parallel to, the underlying substrate.The cantilever structure 10 is connected to the substrate only atsupport regions 30 and 32, where the lateral beam 16 intersects and isintegral with mesas 19 and 20 formed during the fabrication process, oris integral with the sidewalls of a cavity formed in the substrate 18.The cantilever device 10 is conveniently referred to as a releasedstructure.

During the SCREAM-I fabrication process, the beams which make up arm 12,beam 16, and counterweight 26 are lithographically defined in a singleetch mask and the regions surrounding the defined structure are removedfrom the substrate by reactive ion etching (RIE) to form surroundingtrenches which together produce a cavity 34 having vertical walls 36 inthe substrate 18. The cavity also surrounds the support mesas 19 and 20which are formed in the substrate, as illustrated. Thereafter, a secondRIE step undercuts and releases the defined structure and in additionundercuts the support mesas and surrounding substrate mesa. The cavity34 thus incorporates a generally vertical side wall 36, which surroundsthe cavity 34 in which cantilever device 10 is located and a floor 38which underlies the cantilever and which preferably lies in a horizontal(x-y) plane below and substantially parallel to the cantilever 10. Theside wall 36 surrounding the cavity 34 defines not only the supportmesas 19 and 20, but also a substrate mesa region illustrated at 40 inFIG. 1. The side wall 36, as well as the walls of the mesas 19 and 20,are undercut, as illustrated at 41, during the release step. Thesubstrate mesa region 40 may incorporate one or more integratedcircuits, such as those generally indicated at 42 and 44 in FIG. 2,which may be fabricated in conventional manner prior to the formation ofthe cantilever 10.

In the SCREAM-1 process, a metal layer 46 may be deposited on thereleased structure and the surrounding substrate. This metal layer isdiscontinuous at the trenches to separate the metal on the releasedelements from the metal on the substrate both mechanically andelectrically. Furthermore, the undercut portions 41 prevent metaldeposited on the floor of the cavity from being electrically connectedto the metal on the mesas.

The SCREAM-I process is a low temperature process which is compatiblewith conventional integrated circuit processes, and thus withpreexisting integrated circuits on the substrate. Thus, a substrate, orwafer, may first be processed in conventional manner to first provideintegrated circuits which are to be used to control or to sense acantilever sensor structure. A space is left on the wafer for subsequentfabrication of the microelectromechanical structure of the presentinvention by the SCREAM-I process. This process will not damage theintegrated circuits, and the SCREAM-I process provides suitablemetalization to permit selected electrical interconnection between thecantilever structure and the preexisting integrated circuits.

In the preferred form of the invention, the cantilever arm 12 isprovided with a high aspect ratio sensor tip 23 which is formed beforethe above-described cantilever fabrication steps. In one process formaking the tip illustrated in FIGS. 3(a)-3(b), an n-type, P-doped, 18-40ohm-cm, (100) oriented single crystal silicon substrate is subjected toion implantation by an As dose of 5×10¹⁴ /cm² at 30 keV to form an n⁺layer. A dielectric stack consisting of a 20 nm layer of thermal oxide,a 150 nm layer of LPCVD nitride and a 600 nm layer of PECVD oxide isformed on the top surface of the substrate, as illustrated in FIG. 3(a),and a 700 nm film of KTI-OCG photoresist is patterned and exposed in aGCA 4800 DSW wafer stepper, to produce a resist pattern (FIG. 3(a)). Theresist pattern is transferred to the dielectric stack by a CHF₃ reactiveion etch (RIE) to produce a dielectric mask, as illustrated in FIG.3(b). An SF₆ RIE etch then produces a silicon cone under the dielectricmask. In one example, a 4 sccm SF₆ /2 sccm O₂ etch in a chamber having apressure of 5 mTorr and under a DC bias of -300 V with an etch time of 5minutes produced a 1.5 μm high circular cone underneath a 600 nmdiameter dielectric mask. A 100 nm silicon dioxide layer is thenthermally grown on the exposed silicon surface (FIG. 3(c)).

The foregoing was followed by a CHF₃ -RIE to etch any oxide on thehorizontal surfaces (see FIG. 3 (c)), and a Cl₂ -RIE vertical etchproduced a high aspect ratio (height to diameter ratio of at least about10 to 1) silicon post, as illustrated in FIG. 3(d). This etch utilized a50 sccm Cl₂ /2 sccm BCl₃ etch at a chamber pressure of 20 m Torr and aDC bias of -400 V. Thereafter, thermal oxidation was used to sharpen thetip (FIG. 3 (e)). A typical sensor tip may have a height of about 10-20μm, a diameter of about 1 μm, and a tip radius of about 10 nm.

If the tip is to be used as a force sensor or for similar purposes, theforegoing steps are followed by the above-described SCREAM process tofabricate the cantilever 10. However, in some cases it is desirable toutilize the tip as a field emitter, in which case a self-alignedelectrode is formed in accordance with FIGS. 4(f) through 4(h). In thisprocedure, a metal contact window is opened through the oxide layer,followed by an As implant at 30 keV, 5×10¹⁵ /cm², and annealing at 1000°C. for 30 sec. Thereafter, a sputter deposition of TiW covers thesurface of the substrate and the tip (FIG. 3(f)). A planarizing layer ofKTI-OCG is spun on and baked at 90° C. (FIG. 3(g)), and an O₂ plasmaetch removes the photoresist until the TiW above the apex of the tip isexposed (FIG. 3(g)). Finally, an SF₆ -RIE etch of the TiW layer exposesthe end of tip 23, with the tip being surrounded by a closely-spaced TiWelectrode. Thereafter, the SCREAM-1 process is used to fabricate thecantilever 10.

The forces which are to be detected by the torsional cantilever 10produce motion in the z-direction, as noted above, which is translatedby tip 23 and moment arm 12 into a torsional motion about the lateralaxis of rotation 27 corresponding to the lateral torsional beam 16.Vertical motion of tip 23 thus produces a torsional force on beam 16 inthe direction of arrow 50 (FIG. 1), and this motion can be measured by asuitable sensor or sensors such as a plate capacitor, multiple combcapacitors, piezoresistors, or devices such as transistors or diodes.For example, a capacitor for sensing vertical motion of arm 12 may beprovided by formation of a metal electrode 54 on the floor 38 of thecavity 34 below arm 12, as well as an electrode 54' on the floor of thecavity under the 24 counterweight 26. These electrodes may be part ofthe metal layer 46 deposited on the device during the SCREAM processdescribed above, but are electrically isolated from the metal on the arm12 and counterweight 26, as described above. The electrodes 54, 54' andlayer 46 form opposed, spaced capacitive plates which are connectedthrough metal connectors formed by patterning the metal layer or areconnected by wires such as wires 55 bonded to the electrode and tocircuits such as circuits 42 or 44.

The capacitance between the metal layer 46 on moment arm 12 andunderlying metal layer 54 and the capacitance between the metal layer 46on counterweight 26 and the underlying metal layer 54' varies inopposite directions as the arm and the counterweight rotate about beam16 upon the application of a force to tip 23. The rotation of moment arm12 thus produces a differential change in capacitance which can bemeasured with great accuracy to provide a highly sensitive detection ofthe relative motion between arm 10 and the substrate 18.

The metal capacitor electrodes 54 and 54' preferably are fabricated bysputtering metal such as aluminum onto the substrate 18 containing thereleased torsional cantilever structured, and then evaporating aluminum,at an angle sufficient to provide metal coverage under the beams as at56 in FIGS. 4(a) and 4(b), utilizing a rotating stage 57. FIG. 4(b) isan enlarged partial view of the substrate, illustrating how the metal 56passes suspended elements such as individual beams 58 making up thecantilever structure. The metal is then patterned using conventionalphotolithography and reactive ion etching to shape the electrodes and toprovide surface electrical connections or connector pads forinterconnecting the electrodes and adjacent circuitry.

In place of the capacitive electrodes 54 and 54' capacitive comb-typeelements may be provided between the released torsional cantileverstructure and the substrate to sense or to produce vertical motion ofthe cantilever. Such comb-type sensors or drives may incorporatepartially oxidized silicon beams fabricated by the process described incopending U.S. patent application Ser. No. 08/383,524 of Noel C.MacDonald and Ali Jazairy, filed Feb. 3, 1995, and entitled "MaskingProcess for Fabricating Ultra-High Aspect Ratio, Wafer-Free,Micro-Opto-Electromechanical Structures" (CRF D-1689), the disclosure ofwhich is hereby incorporated herein by reference.

In the embodiment of the invention illustrated in FIGS. 1 and 2, the arm12 has a stepped shape of decreasing width and is made up of a grid ofindividual longitudinal and lateral beams 58 having high aspect ratiosto provide the desired rigidity, and thus the high spring constant,needed to provide the desired accuracy of measurement of applied forcesat tip 23. Near the base 14 of the arm 12, the arm is formed with arelatively wide region 60 which steps down at shoulder 62 to a second,slightly narrower region 64. This, in turn, steps down at shoulder 66 toa still narrower region 68 and again steps down at shoulder 70 to thenarrowest region 72 on which the tip 23 is mounted. The stepped,elongated shape gives lateral stability to the sensing arm 12 (in thex-y plane) so that it is relatively unaffected by lateral vibrations.The height of each segment, indicated by arrow 74, is sufficient toprovide vertical rigidity, and to provide a relatively large height towidth ratio, or aspect ratio, for the structure. It will be apparent, ofcourse, that numerous variations of this shape may be used while stillproviding the desired stability and rigidity for the arm.

In the preferred embodiment, the arm 12 is secured to the torsional beam16 by a plurality of longitudinally extending connector beams 76, 77,78, and 79. These connector beams are formed integrally with the arm 12and with the torsional beam 16, and have substantially the same height74 as the arm 12 so as to maintain vertical rigidity and high springconstant. Each of the connector beams 76-79 preferably is thin, forlight weight, with a cross-section having a high aspect ratio; that is,an aspect ratio of about 10 or 20 to 1.

Also, in the illustrated embodiment of the invention the counterweight26 is fabricated as a generally rectangular structure having a heightapproximately equal to the dimension 74 and having an axial length alongaxis 21 and a lateral width parallel to axis 27 sufficient to provide amass which will counter-balance the mass of the arm 12. Thecounterweight 26 is mounted on arm 16 by connector beams 80-83 which aremounted on, and which preferably are integral with, the transversecantilever beam 16, as well as being integral with the counterweight 26.The beams 80-83 are similar to and correspond to beams 76-79, and thushave relatively high aspect ratios to provide the required stability forthe counterweight 26 and to provide a high spring constant mounting forthe counterweight which will permit accurate measurement of forcesapplied to tip 22.

Although the metal layer 46 applied to the surface of the cantilever 10is illustrated in FIG. 1 as a solid covering for the arm andcounterweight structure, a preferred form of the invention isillustrated in FIGS. 2 and 5. As diagrammatically illustrated in FIG. 2,and as best seen in FIG. 5, the moment arm 12 and the counterweight 26are formed as an open grid or lattice work of thin, high aspect ratiobeam segments, such as the segments 58 illustrated in an enlarged viewin FIG. 5. Only a small number of such beam segments are illustrated inFIG. 1 for clarity; however, as diagrammatically illustrated in FIG. 2,the cantilever structure 10 preferably is made up of a large number ofclosely spaced beam segments. The entire structure of cantilever momentarm 12, torsional support beam 16, counterweight 26, and connector beams76-83 is fabricated from single crystal silicon using the SCREAM-Iprocess. Each beam segment thus has a high aspect ratio on the order ofabout 10 to 1, with the segments being released from, and spaced from,the underlying substrate floor 38 and spaced from the side walls 36. Theends of beam 16 are mounted to mesas 19 and 20 at connecting regions 30and 32, where the lateral torsional support beam 16 is integral with thesubstrate to support the cantilever structure 10. The thin, tallsegments provide a high spring constant and high rigidity in thevertical direction and are interconnected to provide a laterally rigidstructure, as noted above.

As described above, vertical motion of the arm 12 in the direction ofarrow 28 produces a twisting motion, or torsional motion, indicated byarrow 50 about the axis 27 of cantilever beam 16 (see FIG. 4). The beamhas a high aspect ratio rectangular cross section, with a springconstant: ##EQU1## where I=ab² /12 is the second moment of area, a isthe width of the beam, b is the height of the beam, and L_(c) is thelength of the moment arm 12 which produces the torsional force (seeFIGS. 2 and 6). For a rectangular beam experiencing a torque T, theangle of rotation θ about its central axis (in the direction of arrows50) is: ##EQU2## where L_(T) is the length of the beam 16 between themounting point 32 and the nearest support beam 79 which produces thetorsional force; G is the shear modulus of rigidity of the material; andβ is a coefficient determined by the aspect ratio b/a. For silicon(100),the shear modulus G is approximately 7.96×10¹⁰ N/m². The following is atable of coefficients β for various aspect ratios of rectangular beams:

                  TABLE I                                                         ______________________________________                                        b/a  1       2       3     4     5     10    ∞                          ______________________________________                                        1β                                                                            0.1406  0.229   0.263 0.281 0.291 0.312 0.333                            ______________________________________                                    

With the torsional cantilever illustrated in FIGS. 1 and 2, a forceF_(z) acting on the tip 23 in the direction 28 at a distance L_(c) fromthe torsional support beam 16 produces a torque T defined as:

    T=F.sub.Z L.sub.C                                          Eq. (3)

The angle of rotation θ is the deflection ΔZ of the tip in the directionof arrow 28, divided by the length of the cantilever moment arm:##EQU3## Substituting equations 3 and 4 into equation 2, the effectivespring constant k.sup.θ_(z) of the torsional cantilever beam 16 is:##EQU4## the factor "2" being included because the specific beamstructure 16 illustrated in FIG. 6 includes two axially aligned segmentson opposite sides of the center of the beam where the torsional force isapplied, both of which are, in effect, rectangular beams mounted ascantilevers to the side walls of mesas 19 and 20.

FIG. 7 illustrates the relationship of a range of effective springconstant values with cantilever length, as a function of the geometricvariables a and L_(c), with the dimension b held constant at 10micrometers. Curves 105 through 108 represent values of beam width aequal to 1.5 micrometers, 1 micrometer, 0.5 micrometer and 0.25micrometer, respectively, where the length of the beam L_(T) is equal to50 micrometers for curve 105 and 200 micrometers for the remainingcurves. These curves indicate that torsional cantilevers fabricatedusing the SCREAM-I process will achieve spring constants between 10⁻³and 10⁻⁷ N/m. It was found that because of the high aspect ratio (b/a)of the beams, they exhibit no curling or buckling.

The theoretical spring constants for a torsional cantilever are shown inthe following table:

                  TABLE II                                                        ______________________________________                                        Force    Deflection                                                                              Spnng Constant                                                                            Ratio to k.sup.q .sub.z                        ______________________________________                                        F.sub.x acting at tip                                                                  Δx                                                                                 ##STR1##                                                                                  ##STR2##                                      F.sub.y acting at tip                                                                  Δy causing a  twisting of  angle φ                                             ##STR3##                                                                                  ##STR4##                                      F.sub.z acting at  center of mass                                                      Δz                                                                                 ##STR5##                                                                                  ##STR6##                                      F.sub.z acting at tip                                                                  Δz causing a  rotation of  angle θ                                           ##STR7##   1                                              ______________________________________                                    

The foregoing table also shows the ratio of the theoretical springconstants to the value k.sup.θ_(z), where v is the Poisson's ratio ofthe material, this ratio being equal to 0.28 for silicon(100). The forceF_(x) in the above table acts on the tip 23 in the x direction to causea deflection along the x axis. Force F_(y) acting at tip 23 produces adeflection in the y direction and a consequent in-plane twisting of themoment arm 12 about support beam 16 through an angle φ, while forceF_(z) acting at tip 23 produces a deflection in the z direction toproduce a rotation of angle θ at the support beam 16. When the forceF_(z) acts at the center of mass; i.e., is directed onto the top of beam16, there will be a deflection of beam 16 equal to Δz. The springconstants of the cantilever structure 10 for these various forces areillustrated in the table. As long as the length L_(c) of the cantileverarm 12 is longer than the length of the torsional supports L_(T), allother spring constants of the torsional cantilever will be one or moreorders of magnitude greater than k.sup.θ_(z). Although the capacitorplates 46 and 54 are provided to sense relative motion in the zdirection, it will be understood that additional capacitor plates orother sensors such as piezoresistors, mirrors, diodes or transistors maybe provided to detect motion of the cantilever in various directions.

Torsional resonators fabricated using the SCREAM technique withk.sup.θ_(z) equal to about 0.3 N/m have achieved Q values of 10⁵ in avacuum. A model of the present invention was constructed in accordancewith FIG. 1, with the beam width a equal to 1.5 micrometers, the beamheight b equal to 12.3 micrometers, the distance L_(T) equal to 50micrometers and the distance L_(c) equal to 950 micrometers. Thisstructure produced a spring constant of 2.58×10⁻² N/m. Furthermore, theresonant frequency of the torsional cantilever was experimentallymeasured at 1.4 kHz. The torsional rigidity was then determined, usingthe following relationship: ##EQU5## where κ is the torsional rigidityand I is the moment of inertia. The known geometry of the device and thedensities of the materials permit calculation of the rotational momentof inertia I=2.55×10⁻¹⁶ kg m². The resulting torsional rigidity isκ=1.97×10⁻⁸ N/m, which means that experimentally k.sup.θ_(z) =κ/L_(c) ²=2.18×10⁻² N/m.

FIG. 8 illustrates a modification of the cantilever device of FIGS. 1and 2, wherein the counterweight 26 is replaced by a second cantilevermoment arm 120 which is a duplicate of moment arm 12 and is coplanar,but extending on the opposite side of support beam 16. The cantilevermoment arm 120 includes segments 122, 124, 126, and 128 which correspondto segments 60, 64, 68, and 72 of moment arm 12, with the cantileverstructure extending over electrode 54' on the floor 38 of cavity 34. Thesymmetric design of FIG. 8 provides improved results for applicationssuch as in a scanning force microscope, since the moment arms of the twocantilevers about support beam 16 are more precisely balanced. However,the asymmetric design of FIGS. 1 and 2 may provide improved results forsome sensor applications such as in an accelerometer. As with FIGS. 1and 2, the electrodes 54 and 54' beneath the structure are used to movethe cantilever or to detect motion, or both.

A still further embodiment of the invention is illustrated in FIGS. 9and 10, wherein the cantilever device 10 of FIGS. 1 and 2 is providedwith electrodes adjacent the torsional support beam 16 for controllingtwisting of the beam under the torsional forces applied by thecantilever arm 12 and the counterweight 26. In this illustration, theposition of the arm 12 and counterweight 26 are reversed. Asillustrated, a first pair of electrodes 140, 142 is placed adjacent end30 of beam 16 while a second pair of electrodes 144, 146 is placedadjacent end 32 of beam 16. As the support beam 16 is twisted under atorsional force, as illustrated in dotted lines at 16' in FIG. 10, theupper edge 148 of beam 16 will approach electrode 142 and the lower edge150 will approach electrode 140. A voltage applied to the electrodes 140and 142, such as the voltage V relative to the voltage on beam 16, willattract the respective edges 148 and 150 of beam 16, causing the beam totwist further, so that the applied voltage will affect the springconstant of the cantilever.

It will be understood that the voltage may be applied to the electrodes140, 142 and 144, 146 by way of integrated circuits fabricated on thesubstrate 18, such as circuits generally indicated at 152 and 154 inFIG. 9, connected by way of conductors 156, 158, and 160, 162,respectively. These electrodes and conductors may be fabricated inaccordance with the SCREAM-I process described above.

It has been found that the planar configuration of the cantilever momentarms 12 and 120 illustrated in the preceding figures may be subject tosome in-plane twisting. In order to sense such twisting, a plurality ofcomb-type capacitor electrode sets 170, 172, 174 and 176 may beprovided, as illustrated in FIG. 11. Each of the electrode sets, such asset 170, may include a plurality of stationary finger electrodes 180mounted in cantilever fashion to a mesa 181 on the substrate within thecavity 34, which contains the cantilever moment arms 12 and 120.Alternatively, the stationary electrodes 180 may be in the form ofmesas, or islands, extending upwardly from the floor 38 of cavity 34.Corresponding interleaved finger electrodes 182 are mounted on thecantilever moment arm 12 for motion with the cantilever arm and withrespect to the stationary electrodes 180. The interleaved electrodefingers form capacitors which may be used to sense the motion of thecantilever arms 12 and 120, both within the plane of the cantilevers andout of the plane of the cantilevers. Further, upon application ofsuitable voltages; for example, through an active feedback circuit (notshown) on substrate 18, motion of the cantilevers can be damped orenhanced to compensate for the twisting of the support beam 16. Thecomb-type capacitor sets 172, 174, and 176 are similar to set 170.

A still further embodiment of the invention is illustrated in FIG. 12,wherein the electrodes mounted on the floor of the cavity 34 in whichthe cantilever structure 10 is located are omitted, and in their placeare provided comb-type capacitors interacting with the cantilever momentarms. In this embodiment, a comb-type torsional cantilever device 200 isfabricated within cavity 34 in substrate 18, again using the SCREAM-Iprocess described above. The cantilever device 200 is mounted on atorsional support beam 16 which preferably is integral with thesubstrate 18 and is mounted on mesas 19 and 20 in cavity 34, in themanner described above with respect to the device of FIG. 1, forexample. The cantilever device 200 includes a pair of elongated momentarms 202 and 204 which are spaced from, and are suspended above, thefloor 38 of cavity 34 and are moveable in a direction perpendicular tothe plane of the cantilever 200, as described above. The cantilevers 202and 204 are mounted opposite sides of lateral torsional beam 16.

The cantilever moment arm 202 extends from a base region 206 which ismounted on beam support 16 by a plurality of connector beams 208. Aplurality of longitudinally-extending, spaced parallel fingers 220 inthe form of high aspect ratio beams extend from base region 206. Thefingers 220 extend above, and are released from floor 38 and thus aremovable with respect to it, are parallel to the longitudinal axis of themoment arm 202, and are coplanar with the moment arm.

Interleaved with the fingers 220 are a plurality of stationary fingers222 which are mounted on mesas 224 and 226 in cavity 34. Fingers 222extend in cantilever fashion parallel to, and between adjacent fingers220 and are coplanar therewith. Alternatively, the stationary fingers222 may be in the form of mesas upstanding from floor 38 of cavity 34and extending between fingers 220.

For symmetry, two sets 228 and 230 of interleaved fingers are providedfor moment arm 202. Two sets 232 and 234 of similarly interleavedfingers are provided for moment arm 204. The interleaved fingers arefabricated with metal layers to provide capacitor electrodes which areconnected to external circuity by way of mesas 19 and 20 for the movableelectrodes and by way of mesas 224, 226 and 224' and 226' for thestationary electrodes. Relative motion of capacitor electrodes 220 and220' with respect to the stationary capacitor electrodes 222 and 222'respectively, can be measured to detect vertical motion of a tip mountedon either of the cantilever arms 202 and 204, in the manner describedabove. Similarly, by selectively applying voltages between adjacentelectrodes, the arms 202 and 204 can be moved, or their responsecharacteristics varied.

Modifications of the comb-type drive of FIGS. 11 and 12 are illustratedin FIGS. 13-15, wherein the moveable and fixed interleaved fingers 240and 242, which correspond to fingers 220 and 222 in FIG. 12, arefabricated by the process of copending application Ser. No. 08/383,524,now U.S. Pat. No. 5,628,917 described above. This process permitsfabrication of partially oxidized released silicon fingers, such aspartially oxidized fingers 240 in FIG. 13, having their upper portions244 oxidized, with the lower portions 246 remaining silicon.Alternatively, as illustrated in FIG. 14, the upper portions 248 of thefixed silicon fingers 242 can be oxidized, with the lower portions 250remaining silicon. A voltage V may be applied across the siliconportions of the adjacent fingers to produce an asymmetric electric fieldbetween them which will result in relative motion of the fingers.Application of a negative voltage to the stationary fingers 242 and apositive voltage to fingers 240 in FIG. 13 will result in an upwardforce on the movable fingers, while the same polarities applied to thedevice of FIG. 14 will result in a downward force on the movablefingers. In FIG. 15, the fingers are provided with alternating oxidizedand non-oxidized segments, the movable silicon fingers 240 includingoxidized (silicon dioxide) segments 252, and the fixed silicon fingers242 including oxidized (silicon dioxide) segments 254. The oxidizedsegments on each finger are aligned with non-oxidized segments onadjacent fingers, so that by controlling the voltages applied to theindividual fingers, either upward or downward motion of the movablefingers can be achieved.

FIG. 16 illustrates another embodiment of the invention wherein twocantilever moment arms 260 and 262 are mounted within the cavity 34 insubstrate 18, in the manner described above. In this embodiment,cantilever arms 260 and 262 both incorporate interleaved sets ofcapacitor plates of the type illustrated in FIG. 12, cantilever arm 260including capacitors sets 264 and 266, and cantilever arm 262incorporating capacitor sets 268 and 270 in place of the underlyingelectrodes utilized in previous embodiments. These electrode sets areused to sense the vertical motion of the cantilever arms 260 and 262 inthe manner previously described. In addition, the device of FIG. 16incorporates sets of stabilizing comb capacitors to prevent or tocompensate for in-plane twisting, cantilever arm 260 utilizingstabilizing capacitor sets 272 and 274, and cantilever arm 262 utilizingstabilizer capacitor sets 276 and 278.

FIG. 17 is a diagrammatic illustration of one use of thetorsionally-mounted cantilever device of the invention in an atomicforce microscope. As illustrated, the cantilever device 290, which maybe any one of the cantilever devices described above, is mountedadjacent to the surface 292 of a sample 294 that is to be scanned. Thesample is mounted on a scanning table 296 which may be driven, as byconventional piezoelectric electric drivers, in an XY plane beneath andparallel to the cantilever 290. The sample should also be adjustable inthe Z direction to move it into proximity with a sensor tip 298 carriedby the cantilever. When the tip is positioned very close to the surfaceof sample 294, the tip will interact with the material of the surface toproduce deflection of the cantilever 290, as is known in the art ofatomic force microscopes. The deflection of the cantilever may bedetected by sensor 300 which produces an output signal through afeedback loop 302 to control the operation of the scanner 296. Thecantilever of the present invention is extremely sensitive and thusallows measurements of extremely small forces with great accuracy.

Instead of mounting the sample for motion, or in addition to doing so,the cantilever device itself can be mounted for motion in an XY plane,with the torsional movement of the cantilever producing motion in the Zdirection. Such an xy mounting is illustrated in the photomicrograph ofFIG. 18 to which reference is now made. In this embodiment, a cantileverstructure 310 is mounted on a torsional support beam 312 in the mannerdescribed above. The outer ends of beam 312 are mounted on a movableframe 314 which is generally rectangular, having four sides, with eachside carrying a set of moveable comb-capacitor fingers. Thus, forexample, frame side 316 carries capacitor fingers 318.

Stationary capacitor fingers, such as the fingers 320 are mounted on anouter frame 322 and are interleaved with fingers 318. The outer framenumber 322 is secured by spring arms 324 to a substrate so that uponenergization of the capacitors made up of interleaved fingers 318 and320, the frame 314, and thus the torsional cantilever device 310, willbe moved along the longitudinal axis of cantilever 310, in a directionperpendicular to support beam 312. In similar manner, capacitor set 330also moves the cantilever device along its longitudinal axis, which maybe referred to as the X axis of the device.

Capacitor sets 332 and 334 are located on opposite sides of frame 314 atthe ends of the beam 312, and when energized tend to move the frame 314along the axis of beam 312, which may be referred to as the Y direction.Torsional motion of cantilever 310 about beam 312 provides motion of thesensor tips carried by the cantilever in a direction perpendicular tothe XY plane to provide Z-direction motion of the tip.

Although FIG. 18 illustrates just a single cantilever mounted within aframe movable in the X and Y directions, the structure can be expandedto provide an array of cantilevers, in the manner illustrated in FIG.19. As there illustrated, a 10×10 array of torsional cantilevers 340 aremounted in a 1 millimeter by 1 millimeter square frame 342, with thecantilevers being spaced at 100 micrometer intervals. Each cantilever ismounted on a torsional beam, such as beam 344 of cantilever 340, betweenparallel frame elements such as the frame elements 346 which areparallel to the X axis of the array. The individual cantilever arms arelocated between spaced, parallel Y-axis frame elements such as elements348. The entire structure is fabricated by the SCREAM process describedabove, and is released from the underlying substrate is mounted on thesubstrate for motion in the x-y plane. Suitable spring arms such asthose described above with respect to FIG. 18 may be provided to securethe structure to the substrate.

On each of the sides of frame 342 are one or more comb capacitor framedriver sets such as those illustrated at 350 and 352, each of whichcontains movable capacitor fingers connected to movable frame 342 andstationary capacitor fingers mounted on a stationary frame or substrate354. If desired, frame 354 may carry multiple sets of stationary combcapacitor fingers which engage corresponding fingers carried byextensions of frame 342. Thus, for example, the stationary frame 354 andthe movable frame 342 may carry comb capacitor driver sets 356, 358, 360and 362. These additional sets of comb capacitors increase the distancewhich the frame 342 can be moved in the X-axis direction and increasethe precision of operation so that the cantilevers 340 can be preciselypositioned. Similar comb capacitor drivers are provided at each of thefour sides of the array, the driver sets indicated at 364 cooperating toprovide motion on the X-axis with similar structures (not shown) on theremaining two sides of the array providing motion in the Y axis.

Each of the sensor tips on the individual cantilevers 340 are connectedto suitable switching and control logic circuits, illustrated generallyat 366 and 368. The connections are by way of the frame elements 346 and348 in the manner described with respect to the movable stage devicedescribed in U.S. Pat. No. 5,506,175 of Zhang et al. As there described,the frame elements are constructed of multiple interconnected parallelbeams with oxidized segments at selected locations to provide electricalpaths between the switching and control circuits 366 and 368 and controlcapacitors and sensors for corresponding individual cantilevers.

A major advantage of the array of FIG. 19 is that an area 1 mm×1 mm canbe scanned by moving individual tips only 100 micrometers along their xand y axis. This allows very rapid scanning of a large area without theneed for large scale motion.

Although the various capacitor structures described above may be usedfor sensing the movement of the cantilever device of the presentinvention, a piezoresistor sensor may be utilized in place of, or inaddition to such capacitor sensors, if desired. FIGS. 20 and 21illustrate such piezoresistor sensors for the cantilever arm 12 of FIG.1, although it will be apparent that similar sensors may be used for anyof the torsional cantilever devices described above. Accordingly, inFIG. 20, the moment arm 12 is shown as connected to the torsionalsupport beam 26 by connector beams 76-79 and the counterbalance 26 isshown as being connected to the beam 16 by connectors 80-83, all asdescribed with respect to FIG. 1. The beam 16 is connected at its ends30 and 32 to mesas 19 and 20, respectively.

In accordance with this embodiment of the invention, a piezoresistorsensor is built into the torsional support beam to measure thedeflection of the cantilever. The piezoresistive effect is the change inresistance of a conductor when it is subjected to a mechanical strain,and in this case, the resistor is defined by implanting an n-type regioninto a p-type substrate (or in the alternative, by implanting a p-typeregion into an n-type substrate) in the region of the beam 16 and mesas19 and 20 prior to fabrication of the cantilever device. The device isthen fabricated using the SCREAM-1 process, as described above, with thetorsional support beam 16 located in the implanted region. The topsurface of the beam and the mesas is covered with an oxide layer 380,and vias 382 and 384 are opened through the oxide layer to expose thesurface of the n-type substrate silicon on the mesas 19 and 20,respectively. Another implant through these vias then defines highlydoped regions 386 and 388 to provide contact areas. Aluminum is thensputtered on to the structure and patterned to provide aluminum contacts390 and 392, in the vias, and these contacts are then connected toexternal circuitry, as by way of wires (not shown) bonded to thealuminum contact 390 and 392 and to corresponding contacts on sensorcircuitry located on the adjacent substrate. The beam 16 then acts as aresistor connected between the contacts 390 and 392.

When a force interacts with the torsional cantilever, a torque isgenerated about the central axis of the torsional support beam 16. Thestress in the beam produces a change in its resistance, and this changecan be detected by incorporating the beam resistor in a Wheatstonebridge in the circuitry carried on the substrate. The measuredresistance change can be directly related to the amount of deflection ofthe cantilever, and thus to the force which generated the torque.

Another stress-sensitive sensor, which is based on the piezojunctioneffect, is illustrated in FIGS. 22 and 23. When silicon devices such asdiodes and transistors are subjected to stress, the characteristics ofthese devices are affected; more particularly, when stresses are appliedto a pn junction, the band gap of the junction decreases and thediffusion length increases. This increases the saturation current andleads to a higher forward current for the same forward bias voltage. Inthe embodiment illustrated in FIGS. 22 and 23, advantage is taken ofthis effect by incorporating a diode into the torsional support beam 16of the torsional cantilever device 10 of FIG. 1. The deflection of thecantilever is then monitored by the change in the current of the diode.

In FIGS. 22 and 23, a pair of sensor diodes are formed by implanting n+regions, such as regions 400 and 402, in a p-substrate silicon at thejunctions 30 and 32 of the torsional support beam 16 with fixed mesas 19and 20. Alternatively, a p+-type regions may be implanted into an n-typesubstrate silicon. After formation of the beam 16 and the mesas 19 and20 using the SCREAM-1 process described above, vias 404 and 406 areopened in the top oxide layer 408 of beam 16 and mesas 19 and 20 topermit contact to be made to the implanted region. This contact isobtained by sputtering of aluminum at 410 and 412 on the mesas and overthe doped regions exposed by the vias 404 and 406.

A second contact to the diode is by way of a p+ region 414 and analuminum contact 416 located on the adjacent substrate 18 at a locationnear to the mesas 19 and 20. As illustrated in FIGS. 20 and 21, the twodiodes are located at the junctions of the torsional support beam 16with the fixed support for the beam. When a force interacts with thetorsional cantilever, a torque is generated about the central axis ofthe torsional support beam 16. The stress in the beam is maximized atthe junctions 30 and 32, and this stress acts on the diodes to cause achange in the current in each diode. The measured current change can bedirectly related to the amount of deflection of the cantilever, andsubsequently to the force which generated the torque. The aluminumcontacts are connected as by wire leads bonded to the contacts, tocorresponding contacts in adjacent circuits to provide the requisitemeasurements.

Although the present invention has been described in terms of numerousembodiments, it will be apparent that variations and modifications maybe made without departing from the true spirit and scope thereof as setforth in the following claims.

What is claimed is:
 1. A compact, low mass force detector, comprising:amicrofabricated support beam having first and second ends mounted to asubstrate; a moment arm extending from said support beam, said momentarm and said support beam being generally perpendicular to each otherand coplanar, said moment arm being comprised of a rigid grid ofmicrofabricated beams and being responsive to a force applied thereto toproduce a corresponding torsional force on said support beam; motioncontrol means for said moment arm; and means for measuring said forceapplied to said moment arm.
 2. The detector of claim 1, furtherincluding a nanometer-scale sensing tip carried by said moment arm. 3.The detector of claim 2, further including a frame supporting saidsupport beam in said substrate for motion in X and Y directions in theplane of said moment arm and support beam, said moment arm being movablein a Z direction out of said plane to produce said torsional force onsaid support beam, whereby said tip is movable in said X,Y and Zdirections.
 4. The detector of claim 3, wherein said frame supports amultiplicity of said moment arms and corresponding support beams toprovide an array of tips.
 5. The detector of claim 2, wherein saidmoment arm is symmetrical with respect to said support beam and said tipis offset from said support beam to provide a high degree of sensitivityto a force applied to said tip.
 6. The detector of claim 2, wherein saidmoment arm is asymmetrical with respect to said support beam to providea high degree of sensitivity to a force applied to said moment arm. 7.The detector of claim 2, wherein said motion control means is located toapply a force to said moment arm which urges said tip out of said planeto enable said tip to track a sample surface.
 8. The detector of claim1, wherein said means for measuring said force applied to said momentarm comprises means for measuring torsional stress in said support beam.9. The detector of claim 8, wherein said means for measuring stresscomprises a piezoresistor in said support beam.
 10. The detector ofclaim 8, wherein said means for measuring stress comprises a diode insaid support beam.
 11. The detector of claim 8, wherein said means formeasuring stress comprises a transistor.
 12. The detector of claim 1,wherein said microfabricated beams in said grid have high aspect ratiosto resist bending motion in a direction out of said plane, said rigidgrid resisting bending motion in said plane, whereby force applied tosaid moment arm produces said torsional stress in said support beam. 13.The detector of claim 1, further including a counterbalance extendingfrom said support beam to produce a torsional force on said support beamwhich is opposed to the torsional force produced by said moment arm. 14.The detector of claim 13, wherein said support beam, moment arm andcounterbalance are a unitary structure.
 15. The detector of claim 13,wherein said motion control means comprises a metal layer on said momentarm and on said counterweight, and a stationary electrode adjacent eachof said moment arm and said counterweight.
 16. The detector of claim 15,wherein said metal layer and said stationary electrode form comb-typecapacitive elements.
 17. A force sensor, comprising:a single-crystalsilicon substrate; a torsionally-mounted cantilever including a momentarm and a support beam formed from said substrate and integraltherewith, said cantilever moment arm being moved about said supportbeam with respect to said substrate in response to a force to bemeasured; and detector means including plural stationary electrodefingers on said substrate capacitively coupled to plural movableelectrode fingers on said moment arm and interleaved with saidstationary fingers for detecting motion of said moment arm.
 18. Theforce sensor of claim 17, wherein said cantilever has a predeterminedspring constant, and further including a capacitor adjacent said supportbeam for adjusting said spring constant.
 19. The force sensor of claim17, wherein said cantilever has a predetermined spring constant andfurther including means for adjusting said spring constant.
 20. Theforce sensor of claim 17, wherein said capacitor fingers comprisepartially oxidized single crystal silicon.
 21. The force sensor of claim20, wherein each of said capacitor fingers includes plural spacedpartially oxidized segments located in alignment with correspondingnonoxidized segments in an adjacent finger.
 22. The force sensor ofclaim 17, further including an integral sensor tip on said moment arm.23. A force sensor comprising:a single-crystal silicon substrate; atorsionally-mounted cantilever including a moment arm, a counterweight,and a support beam formed from and integral with said substrate, saidmoment arm and said counterweight being coplanar with, perpendicular to,and extend in opposite directions from said support beam, and aremovable with respect to said substrate, said cantilever arm beingmovable with respect to said substrate in response to a force to bemeasured to twist said support beam and wherein said cantilever isfabricated from said substrate in the form of single crystal siliconbeams having cross-sectional aspect ratios of about 10 to 1, to providemechanical rigidity in the direction of forces applied to said momentarm; and detector means for detecting said motion.
 24. The force sensorof claim 23, further including driver means for electrically controllingtwisting of said support beam.
 25. The force sensor of claim 24, whereinsaid driver means is a capacitor.
 26. The force sensor of claim 23,further including driver means for electrically controlling motion ofsaid moment arm.
 27. The force sensor of claim 26, wherein said drivermeans is a comb capacitor.
 28. A force sensor, comprising:asingle-crystal silicon substrate; a movable frame carried by saidsubstrate; a torsionally-mounted cantilever including a moment arm and asupport beam, said support beam including first and second ends securedto said frame, said cantilever moment arm being moved about said supportbeam with respect to said frame in response to a force to be measured;and detector means for detecting motion of said moment arm.
 29. Theforce sensor of claim 28, further including multiple support beamsmounted on said frame, each of said support beams including acorresponding moment arm to form an array of cantilevers torsionallymounted within said frame.
 30. The force sensor of claim 29, whereineach cantilever is mounted for independent motion with respect to saidframe.
 31. The force sensor of claim 28, wherein said detector meansincludes a piezoresistive sensor responsive to torsional forces appliedto said support beam.
 32. The force sensor of claim 28, wherein saiddetector means includes a pn junction responsive to torsional stress insaid support beam.
 33. The force sensor of claim 28, further includingdrive means for moving said movable frame in a plane, said cantilevermoment arm being moved in a direction substantially perpendicular tosaid plane.
 34. The force sensor of claim 28, wherein said support beam,said moment arm and said frame are coplanar, said moment arm being urgedin a direction substantially perpendicular to said plane by a force tobe measured.
 35. The force sensor of claim 34, further including drivingmeans for selectively moving said frame in said plane.